1. Field of the Invention
The invention relates to the heating of thin filaments and in particular to the heating of electrically conductive thin filaments for example for chemical vapour deposition or zone refining to produce improved characteristics.
2. Discussion of Prior Art
In the area of chemical vapour deposition (CVD), high strength refractory filaments can be formed by coating fine refractory filaments with high strength materials such as boron and silicon carbide. Such composite filaments have a high strength to weight ratio and are useful as reinforcement materials for plastics, metals and ceramics.
The CVD method commonly used relies on heating the filament substrate to a temperature sufficiently high to cause reaction and deposition from raw materials in the vapour phase. A problem encountered in resistance heating of the filament substrate is in maintaining good electrical contact with the filament to prevent variations in resistance during the electrical heating process. While short lengths of filament could be fixed between carbon electrodes, it is extremely difficult to maintain good electrical connection to a moving filament.
Difficulties have been found when using brass, carbon or other materials as electrical contacts for moving tungsten filaments when silicon carbide deposits, for example, begin to build up. This results partly from the lack of conductivity of the silicon carbide deposit. Mercury has been used by several previous investigators but difficulties due to build up of deposits remain and there is the added difficulty of working with a substance having a toxic vapour. Use of mercury contacts also has the disadvantage that it leads to contamination of the filament by the mercury. The filament substrate is normally fed from one reel to another and attempts to maintain electrical connections at the feed reels rather than between two points on the moving filament has resulted in temperature fluctuations due to the changes which occur in filament resistance.
Induction heating using for example copper coils in an induction furnace cannot be used to heat fine filaments, according to accepted theory, since the skin effect limits the heated volume to a thin layer around the periphery of the wire. The thickness of this layer is inversely dependent on the frequency of the induction furnace and is thicker than the filament diameter for 60 micron diameter filaments for high frequency (HF) induction furnaces; this means that no net current can flow circumferentially around the wire to heat it up. As examples, 450 kHz, 13.5 MHz or 430 MHz induction furnaces would have skin depths of about 0.05 cm, 0.01 cm or 0.0016 cm respectively, for tungsten wire at 1200.degree. C. It is thus necessary for the wire diameter to be considerably greater than the skin depth for successful heating. This argument eliminates the possibility of using the first two frequencies to heat a 60 micron tungsten filament. A further major problem is that the gap between the filament surface and the copper induction coil must be small for efficient heating but of course this gap is very large between a thin filament and even the smallest diameter water-cooled copper coil. Moreover, in the case of chemical vapour deposition a reaction chamber, in which the reactant vapour phase is enclosed, necessitates a large gap between the filament and the copper coil. Thus this problem also eliminates use of the 430 MHz UHF frequency for heating a tungsten filament of 60 micron diameter.
A pluarality of coaxial tuned induction circuits spaced along the length of a wire or filament have been described by DeBolt in U.S. Pat. No. 3,754,112 and by Douglas et al in U.S. Pat. No. 3,811,940. Such arrangements are complicated, requiring individual induction circuits to be tuned--usually quarter wave tuned, and rely on the interaction of electric fields produced thereby to cause a heating current to flow.